The invention relates to a camera tube having an electron source and a target to be scanned on one side by an electron beam emanating from the source. The target has a selenium-containing vitreous layer also containing the elements tellurium and arsenic, the concentration of at least one of these elements varying across the thickness of the vitreous layer.
A camera tube having the features specified in the opening paragraph is disclosed in British Patent Specification No. 1135460.
A problem with vitreous selenium layers is that they are not very sensitive to long-wave radiation. Additions, such as tellurium are therefore often used to improve sensitivity.
In addition, for achieving good operation of the camera tube it is of importance inter alia to suitably block injection of electrons from the electron beam into the selenium-containing vitreous layer so as to minimize dark current, lag and burning-in of an after-image. Moreover, the stability of the camera tube characteristics, for example the stability of the selenium-containing vitreous layer, should be high and the camera tube should be simple to manufacture.
The dark current and the lag, however, may be considerable if high tellurium concentrations are used, which may be the case, for example, when tellurium is present in the whole thickness of the selenium-containing vitreous layer. Moreover, the glass stability of the selenium-containing vitreous layer may be low as a result of the low concentration of arsenic as a glass-stabilizing addition as mentioned in the British patent specification.